Generally, an image forming apparatus of a so-called tandem type has been known. This type of image forming apparatus has a structure in which multiple pairs of photo conductor and transfer unit corresponding to such colors as yellow, magenta, cyan and black are aligned adjacent to an intermediate transfer belt along its moving direction. The developer images of those colors carried on the respective photo conductors are sequentially transferred to the predetermined positions on the intermediate transfer belt such that the color developer image formed on the intermediate transfer belt is transferred onto the sheet.
In the aforementioned tandem type image forming apparatus, the deviation of the transfer position of the developer images corresponding to the respective colors to be transferred to the intermediate transfer belt by the photo conductors causes the color shift in the resultant color developer image. It is, therefore important to perform transfer registration with respect to the developer images of the respective colors. For example, some systems include a technique for sequentially transferring the registration patterns (for alignment) formed of a plurality of marks at intervals onto the intermediate transfer belt by the photo conductor of each color along the moving direction of the intermediate transfer belt. In the aforementioned technique, the deviation of the transfer onto the intermediate transfer belt by the photo conductors of the respective colors, that is, the positional deviation of the respective color images will come out as the deviation between the registration patterns corresponding to the respective colors transferred onto the intermediate transfer belt. Each edge of the registration patterns of the respective colors is detected by an optical sensor disposed adjacent to the intermediate transfer belt to obtain the amount of the deviation of the color images, based on which the timing for transferring by the photo conductors of the respective colors is corrected.
In the technique as described above, the actual deviation amount of the color images is directly reflected as being dimensionally the same as the deviation of the edge of the registration patterns corresponding to the respective colors. In other words, the detection accuracy with respect to the deviation of the edge directly influences the detection accuracy of the deviation amount of the respective color images. In a structure in which the optical sensor detects the deviation amounts of the color images as described above, for example, when the optical sensor is configured to detect the deviation to a maximum level of A mm, the detection with accuracy higher than the level of A mm cannot be realized.